Guide to Fire Alarm System Design

Commercial Security

Metroline Security use our extensive experience to design and install the correct fire alarm system for your premises.

There are different categories of protection within BS5839 Part 1, 2002.  Metroline Security will, if necessary, consult with Building Control Officers and the Local Fire Authority to ensure that the correct system is specified.

Property Protection

The objective of property protection is to ensure that the fire brigade are alerted in the very early stages of a fire.

Type P1: Property protection with automatic detection installed throughout the building.

Type P2: Property protection with automatic detection in designated areas.

Life Protection

The objective of life protection is to protect people from loss or life or injury as a result of fire.

Type M: Manual system with call points.

Type L5: Life safety generally when specific fire engineering solution or where P1 insurance is required.

Type L4: Life safety system, typically a manual system plus smoke detection on the escape route.

Type L3: Life safety system, typically a manual system plus smoke detection on the escape route and heat or smoke detectors in adjacent rooms.

Type L2: Life safety system, same as L3 but detection in fire hazard/risk of ignition such as kitchen, sleeping areas or other designated areas.

Type L1: Life safety similar, as per P1 but where audibility is more critical.

Call Points

  • Call Points should be fitted in conspicuous positions and easily accessible points on the escape route.
  • These call points should be located at the exits to the open air and all storey exits on each floor.
  • One should not have to move more than 45m to operate a call point (25m in any special hazards or high-risk areas).


  • A single zone should not exceed 2,000m² in floor space.
  • Where addressable systems are in place, two faults should not remove protection from an area greater than 10,000m².
  • A building may be viewed as a single zone if the floor space is less than 300m².
  • Where the floor space exceeds 300m² then all zones should be restricted to a single floor level.
  • Stairwells, lift shafts or other vertical shafts (non stop risers) within a single fire compartment should be considered as one or more separate zones.
  • The maximum distance travelled within a zone to locate the fire should not exceed 60m.

Smoke Detectors

There are a variety of different smoke detectors which are identified below:

Optical Smoke Detectors

Optical smoke detectors operate by using infra red light refracting off any smoke particles to trigger the alarm. As a result, optical smoke detectors are more sensitive to smouldering fire often resulting from modern fabrics and furnishings. Unfortunately optical detectors are more prone to false alarms from steam or particularly dusty environments, meaning that they unsuitable for outside bathroom, where building works are taking place, etc).

Beam Detectors

Beam detectors are made up of a transmitter and receiver – the transmitter sends an infra red beam to the RX (receiver). If this beam is in any way obscured then this is detected and the alarm triggered.

Ionisation Detectors

Ionisation detectors work on the principal or charred smoke particles causing a small current flow as they pass between two electrodes. Ionisation detectors are therefore more suitable for fire which are rapidly spread, such as paper and wood. Unfortunately they are more prone to false alarms from burning smells which makes them unsuitable outside a kitchen.

Heat Detectors

Heat detectors comes into two main categories, rate of rise and fixed temperature.

Rate of rise heat detectors respond to a sudden increase in temperature but also contain a fixed element in case of a slow smouldering fire. This makes them most suitable for areas where a smoke detector is undesirable.

Fixed temperature hear detectors have a sensing element which operate the detector when a fixed temperature is reached. These make them ideal for use in kitchens, boiler room, etc where a rate of rise detector would be unsuitable.

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